/Engine/Plugins/Importers/USDImporter/Source/USDClasses/Public/USDAssetCache.h
When importing via the USDStageImporter we will move assets from this cache to the content folder. The problem is that some of those assets may not be currently used (e.g. inactive variant, purpose, etc.). The USDStageImporter will then use these functions before/after translating the scene to know which items from the cache that are actually used for the current scene.
void MarkAssetsAsStale()